Much better in the long run to learn the proper way - with paths etc. but for the PhotoShop challenged - a little step by step for a one off
Rounded Corner Images- Open the image
- Set the mode to RGB (IMAGE > MODE > RGB)
- Created a custom shape sized to frame the image how you want - (use ROUNDED RECTANGLE TOOL)
- Select the custom shapes area by holding CTRL and clicking the layer in the 'Layers' pallet
- Invert the selection (CTRL SHIFT i )
- Select the image layer with just a left click on that layer in the Layer pallet (dancing ants should still be surrounding the rounded rectangles shape)
- Cut away the image corners (Press DELETE or use ERASER TOOL)
- Release the selection (CTRL D)
- Delete the custom shape layer
- Apply a stroke to the image
- Save
Matching Color blending:- Create a background layer
- Color to match the existing sites BG color
- Save for web etc etc
Clear Cornered Gif- Delete BG layers (if any) so checkered work area shows through
- Save for web > GIF > Check TRANSPARENCY > adjust dither, colors, and Matte to best suit the image and site BG
You may have to try a few stabs at getting the edges clean, but in the long run clean edges on rounded corners look very nice.[ Did I forget anything major...? ]