This is most likely ink smearing, not “dirt.” The usual cause is a head strike: the paper is curling upward and touching the print head, which can transfer ink onto the sheet. On small prints like 4x6, curled paper can make this more noticeable.

What to try:

• Check whether the paper is curling; flatter paper helps.
• In the printer driver, set the platen gap to Wide to give the sheet more clearance.
• Clean any paper-contact surfaces the manual recommends, such as rollers, pads, or paper guides, since ink on those parts can also transfer to prints.

A roller/paper-path contamination issue often makes a stripe down the page, while random smudges like yours are more consistent with the paper brushing the print head.

So the most likely fix is: use flat paper and increase the platen gap, then clean the paper path if needed.