I use imageshack all the time to host things quick. I never had problems with them resizing my pictures....
Oh, I assume you're talking about the thumbnail? lolz That's for two things. To preview the image you uploaded, and to provide a clickable thumbnail for forums. But yeah, once you upload an image, just highlight the last option at the bottom(the "direct link" one with just the plain url, right? ), copy it, and paste it into your signature or post or whatever you want. Then add the tags around it...
And yeah, reduce your signature to the sizes given in the forum rules, k?