No probs mate! I'm the same with computers, being self-taught. I've learned more through doing things on this forum than anything else.
So, let me try to talk you through what's needed, given you've downloaded Irfanview. Bear in mind, I hadn't a clue what Irfanview was, or how a picture size was 'measured'! I used to be in the photographic trade, trained in professional photography, and to me, a picture was measured in inches!
Anyway, open the Irfanview programme, then, look at the top left of the screen to 'FILE', left click on that, then left click on 'OPEN'.
You should now have a drop-down list, or a window, where you can select what you want to open. In this case, it should be 'My Pictures'. Click on this, and your picture files should appear.
Select the file you want, and this will then appear as a 'box' full of thumbnails of the pics in that file.
Select and click on the pic you want; if you double click, it will then go onto the main screen, if single click, it will highlight the pic, with a (normally blue) box around the pic. If the latter, then click on 'OPEN', and the pic will then fill the screen.
NOTE you might not see, or recognise all of it, as it could physically be as big as about 24 x 30 inches, depending on the resolution of the camera it was taken on!
Now, again at the top of the screen, left click on 'IMAGE'.
A drop down list will appear.
Left click on RE SIZE / RE SAMPLE.
Another window will appear in the centre of the screen, which might take a second or two to stabilize.
For now, ignore the right half of this window, and look at the left half.
There will be a box, with numbers in blue, at the left, and on its right, another box with numbers in black on a white ground.
This is the size of the pic as it is at the moment, and will be shown in pixels.
(Below this are three 'dolls eyes' with 'Pixels', 'Cms' and 'Ins', for pixels, centimetres and inches, if you want to show actual measurement.)
Just stay with the pixels, and lets say the blue numbers are 3240, and that you want 800.
This means that you want the WIDTH of the pic to be 800 pixels, which is about as big as you need to go for the forum.
Hit the DELETE key, and the two boxes will clear.
Click into the left hand box, and enter '800'.
The right hand box will automatically adjust to the correct proportion, which will be as near as it can get to 600 pixels.
Now, left click on 'OK' at the bottom of this box.
The box will disappear, and the pic will re-appear at the size you have selected.
BUT! If you exit now, the pic will revert back to its original size, so, go back up to the top left of the screen and left click on 'FILE', then left click on 'SAVE'.
Another box will open in the middle of the screen.
Left click on 'SAVE'.
A smaller box will open, with two options, asking if you want to replace the existing file, or not.
Left click on the one you want, which will probably be 'YES'.
The pic is now re-sized and saved.
To do the next one, either go back into file and repeat the exercise above for the next pic, or, on the screen with the re-sized pic, go to the top, and you'll see two arrows, for forward and back.
Left click on the arrow you want, let's say forward, and the next pic will open, again probably so big you'll only see part of it.
Continue as before.
When you've done, and want to post your pics, this is what you do next.
If it's going to be in an existing post within a thread, like this one, look at the two boxes below the reply box, where you type your reply message. The one on the right says 'Go Advanced'.
Left click on this, and a new task bar will appear at the top of the reply message box, which will include the 'ATTACH' icon.
The message you've typed will also be duplicated in a bigger box, as a draft.
Left click on the 'ATTACH' icon, and another box will appear at the left side of the screen. In the top section of this box, there will be 5 smaller boxes, or 'keys', marked 'BROWSE'.
Left click on the first one, and another box will open to show your files - documents, pics, etc.
Select the pictures file you want, then select the first pic you want. Left click on 'OPEN', and a second or two later, the box will 'fill' with the file details. For now, let's just assume you just want this one pic.
Now, left click on the key marked 'UPLOAD'
A message will appear saying 'UPLOADING FILES, PLEASE WAIT'.
When the upload is complete, the 'BROWSE' box will be empty, and in the lower section of the main box, there will be the details of your pic, for example 'jpeg 001 Build27', this being the codes and file name etc that was on the pic in YOUR file.
When that appears, close the box, check that your typed reply or whatever is OK, then left click on 'Post Quick Reply' as per normal.
Your reply, and the pics, are now posted.
If you want more than one pic, just repeat as above, and if more than 5 pics, just upload the first 5, then start again.
For a new thread, it's similar, but the 'ATTACH' icon etc is already there waiting.
This might seem long and complicated, as it's written in full, but once you do it once, it's a piece of p**s, and you'll have no problems.
Hope this has helped!