When you save a graphic in Paint, you have a number of formatting options. Those formatting options are expressed as a file extension (.bmp, .gif, .png, .jpg, or .tif). Understanding what the different formatting options will do to your graphic is important. In most instances, the format that you pick when saving a graphic depends on what you plan on doing with the graphic. Following is a run down that offers some thoughts and notes on these different formats.
To find clipart, visit the Microsoft Images site (http://office.microsoft.com/en-us/images/). This site is literally the only one I trust. Lots of other sites where you find free clipart are littered with viruses and other nasty stuff that you just don't want to deal with.
When you download one of these images from the Microsoft site, it may come in as a WMF, which stands for Windows Meta Format. The graphics are specifically formatted to be used in Microsoft programs like Word. If you open the graphic in Paint, the format is converted to .bmp (BMP). You may well want to do this because you want to alter the colors in the graphic to better conform to the color scheme in your document. I'll talk about actually changing colors later.
BMPs, like all graphics, are composed of tiny dots. The density of the dots increases as you move to a higher-level format. You can see this in action if you open a bitmap and try to save it as a GIF. You get warning messages that the graphic will lose transparency and color...which is a fancy way of saying, "It's gonna be a mess!" If you proceed the graphic pixelates...you can see the dots of color!
A GIF is a mid-level graphic that is frequently used for photographs that are going to be posted to websites. You can convert JPEGs to GIFs without suffering much loss of quality. The point of converting to a GIF is that you get a good photograph with a smaller file size...good thing when you're posting to a website.
A PNG (ping) is a mid-level graphic that is frequently used for graphics that are going to be posted to websites. You can convert BMPs and JPEGs to PNGs without suffering much loss of quality. For the most recent version of Paint, PNG is the default format because it creates graphics with the least loss of quality. Most of the graphics you see on this blog are PNGs.
If you open a graphic that is formatted as a .jpg in Paint, Paint will save it as a .jpg without loss of quality. The problem arises when you take a lower level graphic (GIF or PNG) and try to save it as a JPEG. You can experience a loss of quality. So the rule of thumb becomes scan the original and save it as a JPEG. Convert it to a lower-level graphic if you want to post the smaller file sized graphic to a website.
Avoid using a TIF unless you're a seasoned graphics producer...but then, you wouldn't be using Paint! TIF is the highest-level graphic you can produce (lots of dots per inch and a huge file). I suspect that Paint provides it for those instances when you receive a photograph that is formatted as a TIF and you want to work on it but don't have high-end software. If you bring a graphic in as a TIF, you can save it as a TIF with little or no loss of quality. You can also save it as a lower-level graphic (for example, JPEG or PNG) so that you can print it without using every last drop of ink you have in your printer.
As always, I suggest that you play around with all of these options. You can pick up all of the graphics you need to work with at the Microsoft Images site. If you'd like to read more about these formats, visit Wikipedia. Oh...and remember to have fun!