I have some RSS feeds showing on my website, for speed up loading time I like to cache images directly on my server.
Is it legal to credits the source inside the post and store images from those external RSS on my server?
Much of this depends upon what country you are in. In the U.S., the law is quite clear.
Your scenario does not provide enough information. For example, do you have permission to use the RSS and images even if you do not cache them? The following should apply either way. Hopefully, the answer will be clear.
If you are using the image for commercial use (profit or monetary gain), then you cannot use the image as it is without a license or grant (permission). Get this in writing that carefully spells out the details of use.
If you are not using the image for commercial use, you can use the image without license or grant in a derivative work (not simply modifying the image) and in a transformative work without permission. A derivative work will not apply for your site, however, a transformative work could. It has been determined that a transformative work of an image would be something like a thumbnail image. As far as I know, this is the only transformative work of an image that has been tested legally and survived.
If a thumbnail is not what you are looking for, then attribution would be required along with permission in the way of a license or grant. Attribution is simply giving credit for the work. Often this is both text and a link.
Whether you create a cache copy or simply hot-link the image, you would need to follow the guidelines above.
As well, keep in mind three things: one, the site that hosts the image that you are using, may not hold the copyright and may be bound by license or not using them legally; two, they may not be in a position to grant a license or permission; and three, the host site may require images to be hot-linked for auditing and/or to maintain control over the image itself such as to update it.
If you use any portion of any other site on your site, you will need to get permission and attribution should always be given unless the grant or license specifically states that attribution is not required.