Find a picture of Spider-Man that you want to use for your avatar online.
Save the image to your desktop or a folder where you will be able to find it again.
Open the picture in your photo editing software of choice.
Crop the picture using the crop tool. Most avatars used in forums and blogs are completely square shaped, so make sure that the image has an even length on each side before cropping.
Resize the picture using the resize tool if it is too big for the website where you intend to use it. The majority of forums restrict avatar sizes to 200 x 200 or 100 x 100 pixels.
Save your avatar with a .gif, .jpg or .png extension. Consult the website where you are planning on using the avatar for acceptable file types.
Log in to the forum or blog site where you wish to use your new Spider-Man avatar. Select your user settings, and then navigate to the avatar section to set this picture as your new avatar.