1. Justin will send Susan some picture.
2. Justin will send Susan some pictures.
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Is #2 alone grammatical? What about #1?
3. Some boy wants to see you.
4. Some boys want to see you.
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What about these sentences? Are both okay?
Yes, #1 is not right. The word "some" indicates that what follows needs to be in the plural.
3 and 4 are both correct as long as you intend 3 to mean there's only one unidentified boy, and in 4, there are two or more unidentified boys.