I have known for a long time that a good source of both videos and images is the US government. And, for the vast majority of material that has been funded by taxpayers, it is quite probably in the public domain which means that there is not a copyright and can be used freely. But there is a caveat to be aware of when using these images. The government sometimes pays for images to be used in publications but the image remains in the photographers control and may be copyrighted. So a little digging may be necessary to find out if it is truly in the public domain.
Having said all of that, I did find a United States Department of Agriculture site, the Agricultural Research Service, (ARS), that seems to be totally in the public domain. There are about 2,000 images of flowers, insects, and a lot more here.
But, if you use any of these photos it would be a common courtesy to credit the photographer when that information is available. The lead photo here is of orchids and anthuriums, by Scott Bauer. These images are available in several resolutions and some are quite spectacular. Happy hunting!