The upload size for the gallery is in Pixels.

Say you upload an image that is 2000 x 2000 pixels. You have the option of choosing how large the image is that the viewer will see.  You can select "Resize" which means that Renderosity will display it at a reduced size. If you select "Full Size", the person viewing the image will see the image at it's full 2000 x 2000 pixel size, unless they have set their options to "resize" the image.

You don't need to do anything special to have the image open to it's original size. Renderosity's server automatically does that.

The thumbails that you upload with an image are 250 x 250 pixels in size, to a maximum of 15,000 bytes.

If you are referring to images posted in the forums, Renderosity will automatically resize images that are added to the posts using the little yellow icon inside the reply window.  This is to prevent scrolling due to large images. Those image when clicked on open into another window and if the image was larger than the display size for Renderosity, the image will be full size when clicked on.

Images inserted into forum posts using HTML are not resized and are posted at whatever size they actually are.

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi