Please help me make a "Poster sized" print from Staples

[kingofthenet]

Here is the deal,I have this 1280x768 image I would like to get a 24"X18" poster made out of it from Staples.Now I KNOW that ****ty resolution photo is going to look TERRIBLE blown up to that size, so I use the program RASTERBATOR, First step Defiine paper size in mm, for 24"x18" it's 610x458mm. than it asks me how many sheets am I using(one) than it asks me for max Dot size, I pick the lowest( I want the image to look good close) 1mm in this case, than the program warns me that the file size will get big, thats FINE, I pick in full color(it is about 65mb in size)and a PDF image format excellent I think, it looks great when I open it in Adobe, I bring it to Staples and they are telling me when they try and print it, it won't go(they think it's because of the size of the file) I think it's because they are trying to "stretch it" and all they really have to do is print it natively as it's already SET for that size paper, they are most likely creating files that are 100's of MB's because they are trying to expand it even more.I EVEN REBASTORIZED it with a 2mm dot, and the size of the file went down to like 11mb, but you can see the grain, so I have NO idea how it's going to look printed.Should I just give them the "****ty 1280X768 res original(WHICH looks GREAT on a regular monitor?)and let them Blow it Up?

Please any tips,thought ANYTHING that might help!