(I am going to assume you are downloading ExGFX.) For your ExGFX issue, when you insert the graphics, also have to import a Map16 page that comes with it - these will be the other binary files that aren't the graphics themselves, named Map16Page.bin and Map16PageG.bin. One bin set from a graphics set at a time, you have them in the directory of your ROM, go to a blank page (3-F for foreground; 12-1F for background) and press F3 to load, F9 to save it to the ROM.
Next, there should be a TPL file that comes with the graphics. go to Level -> Palettes -> Import level palette from file. If you have to load two TPLs, load one on another level and copy the needed colors over manually from that level to the one you are working on. In the chance that they use the same palette rows, you will have to change the Map16 data of one or the other to use a different palette row, then paste the needed colors on that one.
You edit Map16 by clicking something, going to Edit #x# attributes (16x16 is for whole tile, behaviors are here, and 8x8 lets you assign some portions of the tile different priority, x- or y-flipping, or different palettes than other portions) and when done, re-paste it over the tile you are editing by right-clicking. Remember - F9 to save Map16 back into the ROM.
It's a bit of a stretch asking for physical assistance with importing the graphics, and hopefully this advice is of some use to you so that you are able to import them without relying on another person.
Anyways... the overworld I see looks to be kind of based on the original Yoshi's Island map, understandable if you're practicing editing the overworld since you're not yet used to the overworld editor. The perspective on the cliff where the ladder is seems to be a bit awkward; the curves at the bottom start one tile off from where they are at the top, and it makes that chunk of land seem slanted as though the pipe is going to slide right off. It would be improved if either the top or the bottom were adjusted promptly, and for an extra aesthetic tweak, add in line creases where the cliff would turn.
There are a few jointed tiles missing as well on some corners of the land, which are a rather negligible cutoff issue - the original SMW itself had many. For instance, at the top corners of the island and the plateau, they are missing, but they're present at bottom left, for a reference. But, that said, there is no major cutoff you should be concerned with - you've got your hills right, the ladder the best it can go with "vanilla" graphics, and overall, it's clean enough for my tastes.
I like what you did with the path design. They smoothly connect level tiles, and none of them are so plainly straight. The ones that make the most sense go around obstacles. It would be really cool if obstacles were added where you think the path could just go straight to a level or a level after, like adding a fence or a small moat around the castle that is only crossable on the west where that path is.
Some time, you should flesh out the straighter portions of the island with various steepness of curves, and maybe you can mold it into something that is not reminiscent of its base without having to scrap it.
Does the pipe lead to an underwater cave, by the way? It seems placed close to the Bowser rock head.
Good luck, man.
Just look above you...
If it's something that can be stopped, then just try to stop it!