How can I have people upload a picture on my form?

You can use the "File Upload" under the "Form Tools" section. You can follow this guide to learn more about it: http://www.jotform.com/help/332-How-to-create-upload-forms. You can also see other suggested widgets for file uploads there.

If you want to limit the file types, like let's say to images only, you can do that by following this guide: http://www.jotform.com/help/29-Upload-Field-Allowed-File-Types 

Please also do take note that accounts have an upload storage space limit, submissions with file uploads will take this space, please check this guide: http://www.jotform.com/help/33-Changing-the-Upload-Size-Limit. Also, please check our pricing page to see the upload space storage limit of each plan.

Do let us know if you need more information on this.