If you right click both images the link is different.
He's right and the image size is different. On another forum I use there is a maximum image size if it is animated. Could be something to do with image size?
Well to add images, you can do a couple fo things. If it's an image hosted on the internet (e.g. on a website or a photo sharing site like Photobucket), when you're making your post, click the little icon above where you type that looks like a tree (insert/edit image). Then type the address of the image in the box that pops up and job done. You can also change the size the image is displayed on the forum here too. The other way is to check "attach image(s)" below where you type and upload your images directly to the board.
To add an image to your signature you'll need to do it in HTML, so you'll need the address on the internet where your image resides. So for example the code for my SomaFM image is this (ignore quote marks at beginning/end):