Unfortunately there is no way to submit a single URL using cURL or anything automated such as what you are suggesting and there is no way to speed up the indexing process for either the content or the images. Neither sitemap submission nor manual URL submission guarantee the content will be added to the index. With the manual fetch it can update the index a lot faster as it doesn't need to be queued, you have done something and it is doing it then returns the results to you so Google kills two birds with one stone and uses the same process to index as required, however the sitemap submission simply goes through the sitemap file and adds each URL it finds to the queue of URL's to be indexed by Google, and Googlebot will index them at the appropriate time based on where they are in the queue and the formula Google uses to decide how frequently to index a given site and a given page.
Unfortunately for your use case you will simply need to keep on submitting the sitemap using cURL unless you want to run of a list and enter each URL manually into the fetch tool. Either way the image indexing won't be any faster as it takes a while for Google to figure out the keywords to apply to an image and the caption and page content is only part of the way Google comes up with the keywords to attach to the image.