There is a common misconception that anything found online is free to reuse; however, this usually isn't true. In reality, most works shared online are protected by copyright and require requesting permission from the copyright owner before being reproduced. NOTE: Even if an image doesn't include a copyright notice it may still be covered by copyright.
Tables and graphs fall under the same copyright guidelines as images. You will need to request permission to use the table or graph. Keep in mind both the table/graph and the data represented are copyrightable.
No. Just because an article was published open access does not mean all of the supporting materials were included in the agreement. You will need to review the open access license for the article to confirm if the graphics and data were included.
Yes, there are several image collections with works available to be reused. Below is a limited list of databases. NOTE: Check the rights statements for each work before using.
CDC Public Health Image Library
Visualizing Health by the University of Michigan