1. When you open the file in Photoshop a dialog box will appear, for best results adjust the image size so the largest dimension is 800 pixels, and change the mode to RGB Colour.
2. When the file opens if it looks like the image below – with a transparent background, just make sure you crop the image so the actual image touches each side (unless the image you are using, fills the entire canvas, then you will need to extend the canvas out by a couple of pixels, so you have a transparent border, if you don’t do this you’ll have trouble editing the image into a logo). If your image has a white background, then you’ll need to follow the instructions from #5 from the next section to remove the background.
3. Now you need to save your image, goto File -> Save For Web. In the dialog box that appears, (#4) select PNG-24, and tick the Interlaced and Transparency boxes. Now click Save and select somewhere easy to find to save your image.
4. Some ai/eps files you open will display a different dialog box (#5), with these types of images, you can not select an image size, you may need to adjust the Crop To option so some of the image isn’t missed.