Resizing images in Paint is a bit fiddly, but if you don't have any image-editing programs handy it'll have to do.
Most image types are supported by forums, but if in doubt I advise you to save your image as a JPEG (.jpg), which is almost certainly supported. Now, the TP forums have a maximum limit of 80 pixels by 80 pixels (80 x 80). If your image isn't that size you'll have to resize it.
Right. First step is to download your image. I can't access yours for some reason, so I'll use the Garry's Mod logo as an example.
Step 1: Download your image. If it's just plain off-the-internet, you can just right-click it and select 'save as', or your nearest equivalent. If it's copyrighted, you probably won't be able to. >.>
Step 2: Once you've downloaded the picture, open it in Paint. Down the bottom of the windowyou'll see the number of pixels your cursor is from the top-left-hand corner. Therefore, if you put your cursor to the bottom-right-hand corner of your image, you'll see how many pixels across and down it is.
Step 3: Use the rectangle-selection tool to select the whole image. Once you've got it, go into the 'image' menu, and 'stretch/skew'.
Step 4: This is the annoying part, which really makes you wish Microsoft updated their Paint program. If you go into the 'stretch' boxes and change the value, you can make it bigger or smaller; 50% will make it half the size, 200% will double the size. From here, tweak the values and resize it a few times until you get it down to 80 x 80, or slightly below. Check how big it is by the aforementioned cursor-in-the-bottom-right-hand-corner. Careful not to rush it, as Paint won't restore pixels once you've removed them; it'll just blur them painfully. Once you have it down to 80 x 80, grab the corner of the 'canvas' and move it to match the edge of your image. Save it!
Step 5: Back to the profile options. Click the button to 'upload avatar from your machine', then choose your newly resized picture. Hopefully that should work.
On the other hand, if it's the right size already, you can ignore everything of the above and just copy the link to the picture and paste it in the profile options.
Sorry for the lack of pictures. My rubbish wireless internet router is playing up again and I have to hold it up and wave it around a bit just to keep connectivity, let alone upload screenshots