When dealing with digital images, file formats are important. All formats are not the same, and some formats are better suited than others. Below JPG and PNG are explained in the simplest terms possible.
Generally speaking the JPG file format is best when used for color photos, or graphics where colors have to blend from one color to another (called a gradient). JPEG stores each individual pixel as a data element inside the file, each data element corresponds to a specific color value. JPEGs can use 16.7 million different colors in the same file! With so many colors to choose from if you had to blend blue to white over a large photo, you can see how you would have enough colors to choose from to make this "gradient" smoothly transition from blue to white.
PNG and GIF are closely related in terms of how they work. GIF was created by CompuServe and used to demand licensing fees. So PNG came along (which was license-free) and competes well with GIF. PNG files are great for Clip Art, line drawings, Web Buttons, and Logos, etc. Basically any non-photograph type of graphic. This is because PNGs save graphic informtion as "areas" of a single color, rather than saving each individual pixel. They CAN save photographic images, but you wouldn't want to use a PNG file for that because of one big difference (from JPG): PNG files can only have 256 colors! This makes PNG unsuitable (typically) for images that require gradient (bluring one color to another smoothly) such as photographs (think of a face blending light skin to darker skin due to shadows).
Redonkadoodles is trying to package up content such that you will receive a PNG version of the image, and a (low compression) JPG version. JPGs can be compressed (by dropping some of those millions of colors!). You will receive the cleanest possible JPG image if that is the format you choose to edit the file in. Compressing JPGs too much will result in blurry/grainy graphics!
HOWEVER You are not limited to just PNG and JPG. If you wish to use other formats, the graphics package you use can probably "Save As" a variety of different formats! To save as your own favorite format, we recommend opening the PNG file into your graphics editor (as that is the cleanest version of the file), and then (typically) choosing "File", "Save As", and then choose the new format you wish to save the file in.
It's easy! Once you download your files and have located them on your computer. Right click the zip file. A popup option will appear. Now click "Extract All..." Choose the destination where you would like to save the files. If this is NOT helpful, and for some reason the option to extract is not available, here are some links to help unzip your files from reputable companies recommended by CNet.