Keith did a painting of the defense of LRT that shows Union troops in backpacks and blanket rolls.
The print you might be referring to is Rocco's "The Defense of Little Round Top by the 20th Maine." I have that print in my collection (A remarque limited edition with mine showing a pencil sketch of a union soldier firing without a knapsack). The print itself has several knapsacks lying on the ground with all of the soldiers--more than a dozen-- fighting without them on. I suspect that Rocco did this for artistic license rather than being used as evidence that soldiers carried them into battle, although perhaps a few did. But it was not the norm, and certainly that particular print does not show them fighting with knapsacks on. In Rocco's "Hold the Ground At All Hazards" done years later, Rocco shows troops coming into position on Little Round Top without any of them carrying knapsacks or blanket rolls. Let me also state that artists portraying Berdan Sharpshooters have depicted them with different shades of green frock coats. Troiani actually located an extant uniform and had it tested to determine the correct color which, obviously, might have been affected by the oxidation over time. Anyway, we now realize that the color was not actually the forest green seen on many toy soldier representations of that unit (although I do have some figures that have incorporated Troiani's findings in their color renditions).
Finally, in recent years research has disclosed that the sharpshooters had placed their green frock coats in storage just after Chancellorsville and before the Battle of Gettysburg. They wore regulation blue sack coats in their place. Once this information came out, I could no longer portray a Gettysburg scene with Berdan's toy soldiers wearing green frocks, regardless of the shade. This also affected my role as a sharpshooter reenactor working as a volunteer for the National Park Service at Gettysburg. The unit with which I was associated had struggled with the choice of wearing blue sack costs during presentations for this event specific occasion or wearing green frocks as a means of educating the public about the sharpshooters generally to show how they were distinctive. We solved that problem by wearing both. I think that most toy soldier hobbyists simply do generic scenes and are quite content to make idealized dioramas instead. There's nothing wrong with that and I have done that as well; but for something more specific, utmost accuracy is important. My caveat mentioned previously about using artists' renditions as a substitute for one's own research into primary source materials still stands.
Who knows? Maybe some day further evidence will come out that every soldier fighting in Strong Vincent's Brigade on Little Round Top had their knapsacks and blanket rolls with them. If so, all of us who said to the contrary, including the artists, will be proven incorrect. But until that time arrives, let's have a go at at least giving us the option to make our displays more convincing, if we want to, by having figures without them.