You're looking for something to export the image at a lower resolution (resize) rather than something to compress the image. Any open source gallery/image editing app should be able to do this.
One example would be "image toolbox" on FDroid. I've not used it but it seems to have the feature you need
Jan.ai might be a good starting point or ollama? There's https://tales.fromprod.com/2024/111/using-your-own-hardware-for-llms.html which has some guidance for using jan.ai for both server and client