The file sizes for photos vary greatly, depending upon the cameras used to take them, the settings used by those cameras and the image format used.
As a rough guide, images downloaded from websites tend to be around 0.25Mb in size, such as this one:
http://img16.rajce.idnes.cz/d1602/9/9542/9542246_72ce83967c4c077c930431ba7f2bd527/images/P3110431.jpg
You could get about 16000 of those of a 4Gb memory stick (which is the smallest size that most supermarkets now sell). So almost any memory stick would meet your needs for such photos.
Photos taken on a 4Mp camera (using the highest available resolution and the default jpg format) tend to be around 1.2Mb. A 4Gb memory stick would still be big enough for your needs for such files, as it would hold about 2800 pictures.
To get the same number of photos stored if they were taken on an 8Mp camera (with the same settings) you'd need an 8Gb stick.
For other cameras/file sizes, see the first table here:
https://kb.sandisk.com/app/answers/detail/a_id/69/~/number-of-pictures-that-can-be-stored-on-a-memory-device