Different filesystems for different operating systems work differently. But no matter which filesystem you use, this is unlikely to overwrite the data.
One thing you need to understand about filesystems is that the friendly directory- and filenames are usually an abstraction for the human user. On the actual hardware, all the file data is one big lump of one's and zero's. In order to find a file, there is a file table which maps human-readable file paths to hard drive locations and lengths.
I batch rename them 1-100.jpg, I then change the extension of all the files to .txt.
This will change their names in the file table, but not change any of the data.
I then make 100 blank text files and over write the converted image files.
This will truncate the length of all the files in the file table to 0 bytes. There is no reason for the filesystem to overwrite the actual data. You will likely end up with lots of space marked as empty, but which still contains the image data until the filesystem needs the space for new files.
I then split the 100 blank txt files and split them 50/50, renaming them both the same and over writing, then split to 25/25, rename and over write, till I get just one file. Then I just delete this file.
I am not really sure what you mean with that, but it sounds like something which would again just affect the file table entries but none of the actual data.
If you want the filesystem to actually overwrite the data and not just the filename, you usually need to replace the content of the file with new content of the same length and then save it under the same name. So the manual way to shred a file would be to open it with a text editor and replace the content with the same number of random keystrokes (you can use copy&paste if you like). This is of course very tedious, so you might want to use one of the many file shredding programs which are available for free.
Also be aware of the special intricacies of shredding files on solid-state-drives. Their wear leveling algorithms create another layer of indirection below the filesystem which the operating system is not aware of and might result in data not actually being overwritten on the hardware even when the filesystem tells the hardware to do exactly that.