1)
Okay, so I filled in my worksheet in blue, and recolored a Nintetales, which is now pretty. Step 1 is to select around the Nintetales and hit Ctrl+C, or Copy.
2) Open up GIMP, hit Ctrl+V, or Paste, and it should look like this now.
3) Click Layer>Transparency>Add Alpha channel, like in he picture.
4) Now click this little tool button. The Select By Color Tool.
Then click the blue and press Del or delete.
5) It looks a bit like this now.
:/ I don't like that fuzzy outline, so now click Select>None, like in the pic
6)Now click Image>Autocrop to get rid of all that unnecessary transparent space.
7) It should now look like this
:D
8 ) Click File>Save As and name your file and then write
.png
If you don't, the quality will get screwed up.
Save it where you want.
9) Now, upload it onto a image hosting site, like Iaza, Photobucket, Tinypic etc.
10) Copy the BBCode URL it gave you and paste it into the text box of the forums.
11) When you hit Preview/Submit, it looks like this
:D Well done, you transparentized a sprite on GIMP :D