Here you go:

1. Open the image in photoshop.
2. if it's a black and white image, make sure it's in grayscale mode, not RGB or CMYK. (Go to Image -- Mode -- Grayscale)
3. Open up image size ( Image -- Image Size) and check on the resolution and image size -- if it's bigger than it needs to be, cut the dimensions. Generally, nothing for the web usually needs to be bigger than 700 px (or absolute max 1000 pixels) on the longest side.
4. Go to File -- Save For Web (or some versions of photoshop have File -- Save as Optimized)
5. Here's where you can fuck with the compression and file format of the image. I usually have it display the image in the 2-Up format (look at the tabs at the upper left hand side of the screen.) That way I can see the original image and compare it to how it will look when I compress the resolution. For most stuff, you want to save it as a JPEG, and then you can slide that little Quality slider down until you get the file size you want. General e-mail etiquette is don't send anything over 100K if you're wanting really nice quality, or 50K if you can settle for less nice preview quality. If you've got to send something bigger than that, it's really good to use YouSendIt and not clog up people's inboxes.