custom media image crop size??

hi, is there a possible way to generate a default size of image when user is uploading an image?

I'm not talking about setting the sizes on Setting >> Media , I just to have a custome size of 100x100 thumbnail when user is uploaded his media image. so that the generated custom file I can show it on my main wpmu site.

so usually the sizes declare on media is for user and for me I want to have a custom size just to generate only for my main site and not on my members.

thanks TIA