First, make sure that the clip art is in RGB 24 bit color before you make any changes to it. In Photoshop, select Mode/RGB. In PaintShop Pro, select Colors/Increase Color Depth/24 bit. You will want to change the colors to 24bit for almost any manipulations that you do to clipart.
How big should a web graphic be? Well, the most common screen display resolution is 640x480 pixels. You'll want to allow room for the edges of the web browser, so you won't want your graphic to ever be much wider than 500 pixels across. If your graphic is wider than the monitor of the person viewing it, they will have to scroll to the right in order to see the entire graphic. So, remember to work in pixel measurements for web graphics. What's the DPI of a monitor? It varies. I use 96 dpi when I work on web graphics, but the graphics will display at the viewer's monitor resolution in a web browser. I don't know why this seems to be the most difficult thing to explain about web graphics, but pixel size of an image matters - DPI doesn't. You can save a graphic as 72 DPI or 300 DPI - it will display exactly the same if you're using the same monitor to view it.
To recolor the clip art in Photoshop, select Image/Image Size. Make sure the units of measurement are set to pixels, and leave the Constrain Proportions checkbox checked, so that your image isn't squished horizontally or vertically when you resize. Change either the vertical or horizontal pixel measurement, and Photoshop will adjust the other measurement accordingly.
What about the dots per inch measurement? Well, web graphics display at the resolution of the web site visitor's monitor, so the same image will appear very, very different on different monitors. Monitor resolutions vary widely--from laptop screen size to the new 30" monitors. For the sake of resizing images, set the DPI to 96 pixels per inch.
To resize clip art in PaintShop Pro, select Image/Resample. Make sure the Maintain Aspect Ratio radio button is selected. Select Custom Size, and change one of the measurements (vertical or horizontal). PaintShop Pro will recalculate the other measurement.
To save the resized art as a gif file, in Photoshop 3 select Mode/Indexed, in Photoshop 4 select Image/Mode/Indexed, and then save the file as a .gif file. In PaintShop Pro, select Colors/Decrease Color Depth, and select a color depth for the image. Then save the file as a .gif image.