I have Seven hand drawn and photo images that need to be converted to 300 dpi and jpeg/png format in order to go to a publishing company

My husband has written his 2nd book, and we have some hand drawn pictures from our young granddaughter that we absolutely want in the book as illustrations, the publishing company has been giving us fits over the fact these images need to be 300 dpi and they need to be jpeg or png format. I have exhausted every avenue that I know of, a friend of a friend gave this website as an option and that I could done with you what I need for the publishing company. I have 6 images that I need worked over whether it will be a new layer (my apologies I do not understand this stuff. Way over my head) or whatever in order to meet the publisher's demands. 300 dpi and jpeg or png. I need this completed ASAP. We are running way behind on this project to get it to the editing department. Whatever you can help me with, at the lowest cost possible would actually be a lifesaver. Please let me know ASAP . Thank you in advance. Linda