Here is a very quick way to create good looking raised images for web use, etc.
1) Create a new image with a transparent background
2) Draw a shape using vectors, or similar, and fill with your choosen colour; ensuring that outline and fill are the same.
3) Convert the layer to a raster layer.
4) Duplicate the layer.
5) Select the first layer again and go to Effects / 3D Effects / Cut out. Set the horizontal / vertical settings to something like -5 (to begin with; play with the settings later!), set the shadow colour to something that is darker than your shape's colour. Check the 'fill with colour' box and select your shape's original colour as the fill. Next set the opacity to 75% and the Blur to 15% (the more blur the 'rounder' the image). Now hit 'OK'.
6) Go to the duplicated layer and, again, go to Effects / 3D Effects / Cut out. This time set the horizontal / vertical to +5 and select a lighter shade than your original colour for the shadow. Un-check the 'fill with colour' box and set the opacity / blur as described above. Hit OK.
7) Now merge the two layers together and there you go.
It's not perfect, but it's a very good starting point from which to use your lightness up / down (etc) brushes.
For a first attempt try this with two or three overlapping green circles or something similar. Use a medium shade for the circle colour, a dark green for the shadow (step 5) and a light green or white for the highlight (step 6). Play around with the settings for a bit and you can get some great effects with this; more importantly (for the web anyway) it is fast and easy to repeat.