How did you force it into RPGM? I'm also wanting to use it with RPG Maker!
It might be harder in some other programs like photoshop with their weak pasting system for selected 48 by 48 grid but it's pretty easy to set something up in a 768x768 in Corel Paint Shop Pro. Set up the grid. Copy using the marqui and with snap to grid set up and paste them with corresponding rectangular shaped selections. Tried to explain it really fast looks a little confusing.
Yeah I would highly recommend getting Corel Paint Shop Pro. Just one payment so therefore no monthly payments. There is this problem that some stuff such as the mz, dark rpg, and mv trinity tilesets all have a black background in Paint Shop Pro. What you do is you open it in photoshop then copy the entire thing and create a new file the same size 768 x 768, goto it and select "paste in place". From here you want to goto from along the top "File" --> "Export" --> "Export As" and either override the old file by exporting this over the old file (which I prefer) or name it what you wish. The Dark RPG, MV Trinity, or MZ tilesets, characters, side view battlers, weapon sheets, icons, and more will always come with that black background as will a few others. It's a bit of a pain. I got photoshop to fix this problem because it comes in with the black background in Corel Paint Shop Pro but not in Adobe Photoshop. But I need Corel Paint Shop Pro to have a perfect copy and paste grid pieces where in Adobe Photo Shop they let you use "paste into" when using the grid and it's pastes it off instead of exactly as what and where was copied within the grid space from the other grid. In Corel Paint Shop Pro you must goto "View" --> "Grid" and goto View --> "Snap to Grid" there have both Check-marked (on) and under "View" --> "Grid, Guide & Snap Properties..." You will notice you can have the horizontal grids and vertical gridswith independent values, which is ANOTHER reason I need Corel Paint Shop Pro. When single weapon sheets need to be copied and pasted into a new image from a twelve weapon sheet. The twelve weapon sheet need to cut a single weapon from it this requires a "Grid, Guide & Snap Properties..." to have a 288 horizontal grids and 64 vertical grids which is not an even square grid but rather a rectangle. These single weapon sheets may be required for special weapon animations like the once were in MV. Once you have the size selected 48 hor / 48 vert for tilesets (32 hor / 32 vert for icons). Just select the box / boxes you want to copy and and select Edit copy them and on another page ( or the same i suppose ) select an empty space spaces equal to the squares copied ( 1 typically ) and select "Edit" --> "Paste Into Selection". This will copy and paste a perfect copy in the square with perfect placement to where in the grid space it was before. No more tedious nudging some icon into place pixel by pixel to get the image, needing a grid, to hopefully find the correct spacing. Anyways hope I helped. I just didn't finish doing this one (The Chimerical World Map) because I have not really want to and made 768 x 768 sheets of what I needed from The Chimerical World tile and also as there are tilesets piling up as new tilesets come out with excellent quality and some not so excellent that I can use all formatted for RPG Maker MV/MZ.