As there are 1000MB in 1GB, if your pictures are (for example) each 4MB in size, you'd need a 4GB car to hold them all.
Assuming that your pictures are in the JPEG format (as most are), rather than being something like RAW files, if they've been taken on a fairly basic digital camera, or with a fairly basic mobile phone, they might have file sizes of around 1MB or 2MB each. If they've been taken on a top-of-the range digital camera, or on an expensive mobile phone, they might have file sizes of around 8MB.
In practice, the smallest size memory stick sold in many shops these days is 16GB, which should easily be able to hold 1000 JPEG files (irrespective of the device that they've been taken on).
https://groceries.asda.com/product/data-storage/sandisk-cruzer-blaze-16gb-usb-flash-drive/910000422630