You dont need to upload two different sizes because the wiki uses imagemagick to resize pictures and you can use wiki syntax for creating thumbnails in your desired size. mutante
like:
[[Image:Wikipedesketch1.png|100px]]
or [[Image:Wikipedesketch1.png|thumb|The Wikipedia logo]]
http://en.wikipedia.org/wiki/Wikipedia:Picture_tutorial
http://en.wikipedia.org/wiki/Wikipedia:Picture_tutorial#Resizing
