Where are the JPEGs?
PNG files (.png) are better suited for simple images like logos, and can be used wherever you would use JPEGs (.jpg). You can read more about the difference between image file formats here.
Can I use these logos?
Yes, they are Creative Commons liscensed, see full information below.
350.org Logo by Matthew Anderson is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.