As for the database, I would suggest something like this
table images
int `index` auto_increment primary key ”gives you ~4billion indexs
text `path` ”path to file
varchar(40) `hash` ”md5 has of file for unicity (thus you dont get double uploads)
text `tags` ”holds the tags
int `poster` ”holds user id of poster
int `date` ”date image is submitted
int `status` ”a status variable? flagged? approved? deleted? archived? etc.
”add what ever you want.. etc.

table tags
int `index` auto_increment primary key
varchar(120) `tag`
int `count` ”counts instances of tags, (this table is for quick access of amounts w/o massive searching

table comments
int `index` auto_increment primary key
int `image` ”images index that the comment belongs to
text `comment`

table logs, query_cache, preview_images, etc (lots can be done to speed up the queries at the expense of memory and harddrive space)

For those of you who are quite versed in sql, please feel free to correct me. I’m sure there is something I could be doing better (like using blobs instead of text fields)!