In this sentence:
You can send e-mails to targeted photo buyers and include a link to your website.
If I change the word 'targeted' to 'target', will this make any changes to the meaning of the sentence? Or, will it be grammatically incorrect?
If you change targeted to target, the sentence will remain grammatical—at least so long as it's parsed in an appropriate way.
This is a grammatical parsing of the sentence (the words in parentheses do not need to be there; I have added them only to clarify the interpretation).
✔ You can send e-mails (in order) to target photo buyers and (you can) include a link to your website.
By changing targeted to target, you change the adjective to a verb. This does change the meaning; it adds additional information about the intention behind the sending of the emails. But it doesn't make the sentence ungrammatical.
Note that it's also possible to interpret target as an attributive noun. (In other words, a noun that acts as an adjective.)
In other words:
You can send e-mails to target photo buyers.
→ You can send e-mails to buyers of photos of targets.
I don't personally think this would be as common an interpretation (at least I find it a bit awkward) but it's still possible.
It's also been pointed out to me in a comment that target photo buyers could be used in the same sense as target audience.
To me, that's the least likely interpretation, but it's not impossible. While target audience is a common set phrase, using target X (where X is a different noun) is not nearly as common.
Even that change would be grammatical. And while it wouldn't change the meaning nearly as much as the others, a target audience is a bit different than a targeted audience, and, by extension, so would target photo buyers be different than targeted photo buyers. One refers to a past tense action (although it's still an adjective) and the other is a simple attributive noun.