Afaik, no. Images are processed in three different ways on Tumblr.
Photo / Photoset / Panoramic Post
When creating a new Photo / Photoset post any images added, Tumblr will process, and create multiple copies at various sizes. You can then retrieve a specific size by using the appropriate theme operators:
{PhotoURL-HighRes}
{PhotoURL-500}
{PhotoURL-400}
{PhotoURL-250}
{PhotoURL-100}
{PhotoURL-75sq}
Theme Assets
You can also upload an image whilst customising a theme, via the customisation page. Unlike Photo / Photoset Post, these images are processed and retain their original size and multiple sized versions aren't created.
Post Body / Caption
Finally you can upload an image whilst adding content to the post body / caption. These images are processed and capped at 500px high / wide.
Hope that helps!