Load your image file in photoshop and double click the background layer on the Layers panel on the right hand side. Name the new layer whatever you and keep default settings.

Now, duplicate this layer by right-clicking or dragging to the create new layer button at the bottom right of the screen (figure 1).

Select the first layer and choose the erasor tool from the tools panel. Erase everything from this layer (to see what you're doing click the eye button next to the top layer to hide it).

Select the top layer again. Go to the tools panel and select rounded rectangle (figure 2). Select the 'paths' tool type (fig 3) and draw a rectangle from the top-right to the bottom-left of your picture.

From the paths panel (next to the layers and channels tabs) right-click and on the work path and choose Make Selection. Accept default settings.

Press Alt-Shift i to invert the selection.

Go back to the layers panel, from the drop down menu next to opacity choose normal and click the Add Layer Mask button at the bottom of

the layers panel. You should now see the rounded edges with transparent checker grid underneath.

To save images with transparency for the web goto 'Save file for web & devices' from the Files menu. Choose a 24bit PNG file format (for high quality). For smaller files choose PNG-8 or GIF file formats. If you want a coloured background, fill the background layer with the desired colour and save as a JPG