Best bet is to get some paper and draw the guides and folds with pen and ruler. Cut and fold it to see if it works for you.
If it does, input those measurements to the computer screen.
I actually can't follow what you're asking, so best that you work out at template by hand first before turning to the computer. That's what I've always done.
You just did ask.
Anyway, I'm not quite sure what you mean by placing images on the fold. It's possible to have a fold at the edge of an image or at a color break between panels -- I do this for some clients -- but if you do you must plan carefully, which means you need to know in advance where the folds will be, and you should also consider what will happen if the fold doesn't fall exactly where it should (folding equipment and cutters are sometimes very slightly out of adjustment). On the inside of your brochure a slightly offset fold is not terribly noticeable, but on the outside it will show the edge of the adjacent panel. It's fairly common to actually bleed what will be the "front" panel (the one that readers look at first) into the area of the back panel by one or two points and intentionally leave a little of the front showing on the edge of the back, just in case.
As for where the folds belong, that's open to discussion, and it's a discussion you should be having with the printer. It's normal, however to have the panel that folds inside to be narrower than the panel it folds against. One common setup for 8.5 x 11 three panels is the two outside panels at 3.6875 and the one that folds in at 3.625, but your printer may want more of a difference. And don't forget that the narrow panel is on the left on one side and on the right on the other.
And, you just did answer! And I'm glad you did, always good receiving your replies. Many thanks for your answer!
You're welcome. I didn't even notice it was you or I'd have said "Hi."
Definitely ask the printer about the panel widths, then fight with them if you think the difference is too great. There's a fairly wide range that will work. One bindery guy I used to know preferred very little difference -- he said a big gap was more likely to dog-ear in the folder in his experience, but you do need a little to accomodate the paper thickness in the fold. I have an old and worn-out dessktop model Baum folder here that I use to do brochures I print on the laser. I layout with even columns, usually, for these, but always have a gutter of at least a quarter inch that's clear, or a design that doesn't need to break precisely on the fold when I use this machine so I can disguise the slight differences in the panel widths.
The last critical fold job I did the printer gave me panel dimensions and insisted that I reduce the overall width of the layout by a few thousandths of an inch (it was a rebuild of an old job someone else had designed and printed and started as 5 4-inch panels), and I ran my breaks just over-width as I mentioned before, but nobody, apparently, told the folder operator, even though there were fold marks and notes in the slug area. He ended up adjusting so the folds were exactly on the breaks, and it was OK, but I did a lot of adjustment I didn't need to to set it up correctly.