Here is how you can make you icons in Paint:
Getting the border
Open wc3viewer, which you can get in the files download section if you don't have.
Click the BLP option in the little side window.
Click the +'s to get to ReplaceableTextures\CommandButtons\
Click any of the buttoms you see there.
Go to Current Files->Extract (and convert) BLP File. Choose to save as a BLP.
Open the BMP in paint.
Click the zoom tool and zoom to to 8x.
Use the rectangular selection tool to select everything but a 4-square border. This should leave the metal thing
Press Delete.
Zoom back out.
Putting it together
Find your picture using google. For this example, I picked this:
Right click on it and select copy.
Open a new copy of Paint.
Go to edit->paste.
Next, you have to make it be 64x64 pixels, the size of a Warcraft icon. Paint doesn't make this easy.
Click and drag the bottom-left box of your selection to make it be just about exactly 64x64 pixels. First, get a rough idea by looking at the border you made above. Next, select everything (CTRL-A) click and drag on the bottom right corner of your selection, making it smaller. The deselect everything (by clicking on the rectangular selection tool) and drag the bottom left of the canvas (white area) up to match with the edges of your image. Go to Image->Attributes to see the exact size. Keep resizing until you get to somewhere between 56x56 and 64x64. If you got to less that 56x56, then undo and try again. If you got to between 56 and 63, then expand the canvas to 64x64 in attributes and drag your image to the center, leaving white around the edges.
Go to your border image. Press CTRL-A, CTRL-C.
Go to your image image. Click the rectangular selection tool and choose the second of the two options that are available below.
Press CTRL-V. The border should now be around the edges.
Save as BTNTheButton.bmp or something.
Converting
Open wc3viewer again.
Click the convert BMP/TGA/JPG to BLP button.
Select the BMP you saved.
When it asks you if you want to add an alpha channel, say yes.