Though better than nothing I wouldn't really recommend a flash drive for anything other than short term backup (i.e. they are OK if you use more than one on a daily rotational system of backups)
For long term backups they are not reliable enough. you would be much better burning your pictures to DVD or CD, make two copies on decent quality media and store them properly, preferably in separate places and if you need to look at anything in the backup only use on copy, leave the other totally untouched.