Also consider that with flash memory in general nowadays, including SSDs, just overwriting a file won't cut it.
If you overwrite a certain file the controller doesn't write in the exact same physical locations (blocks, sectors, cells, whatever you call the location on the memory chip).
This is for various reason, such as NAND flash (ie. in SSDs) can only reliable write to empty cells without any data (so would have to do a slow erase operation first), and also to ensure the life-time of the chip is extended by not writing the same location again and again but evenly spread write operations on all cells throughout the lifetime...
In other words: if you have a nude pic of your girlfriend and the file is on cells 1001, 1002 and 1003, the overwrite process leaves 1001, 1002 and 1003 untouched and marks it for garbage collection later. It may write to 2001, 2002 and 2003 instead. If you come in now with some recovery/forensic tool you can recover the nude pic, although you think it has been overwritten. Only way to really overwrite is to also overwrite the free space on the disk/memory.
Edit: Since SD cards, USB sticks etc all use NAND flash memory, it also applies to these. There are tools that let you shred (overwrite) files and empty space. Not sure if you can get some tools specialized for flash memory nowadays, but I assume you can.