You have a few options.
If the graphic is one ready for "stretching" you can set borders in the advanced panel, then write a script that will make it grow from 1 pixel to more in the dimension(s) you want.
You can also have an object the size of the exposed panel, but with no graphic assigned. Set your image object as a child of the frame object, then move it via script from outside the area defined into the area defined. Meaning, say Object.Top = -128 changes to Object.Top = 0 through 16 8-pixel steps.
If the main graphic that you want to slide out from is large enough, you can simply hide your panel behind it, and move it out from under.
I think your best solution will depend on your particular needs.