How to move files from a lot of folders into a single folder without moving the folders themselves?
This is what you're looking for ?

mv *.jpg Low-Resolution\ Pics/ Moves all '.jpg' files in current directory to low res folder.

mv WashingtonDC00*/*.jpg ./High-Resolution\ Pics/ Moves all '.jpg' files in folders beginning with 'WashingtonDC00' into High res folder.

Files before:

.
├── High-Resolution\ Pics
│   ├── WashingtonDC002.jpg
│   └── image0.jpg
├── Low-Resolution\ Pics
├── WashingtonDC005.jpg
├── WashingtonDC006
│   ├── WashingtonDC006.jpg
│   ├── WashingtonDC0061.jpg
│   ├── WashingtonDC007.jpg
│   ├── WashingtonDC008.jpg
│   └── rubbish.txt
├── WashingtonDC007.jpg
└── WashingtonDC009
    ├── WashingtonDC009.jpg
    └── rubbish.txt


Files after:

.
├── High-Resolution\ Pics
│   ├── WashingtonDC002.jpg
│   ├── WashingtonDC006.jpg
│   ├── WashingtonDC0061.jpg
│   ├── WashingtonDC007.jpg
│   ├── WashingtonDC008.jpg
│   ├── WashingtonDC009.jpg
│   └── image0.jpg
├── Low-Resolution\ Pics
│   ├── WashingtonDC005.jpg
│   └── WashingtonDC007.jpg
├── WashingtonDC006
│   └── rubbish.txt
└── WashingtonDC009
    └── rubbish.txt