How to find the settings of WhatsApp in my smartphone?

This has nothing to do with your having no memory. There are a few methods which may work in your case.

1.Restart your Phone
You might have already tried this option. But basically most of the memory related issues are solved by a reboot. Restarting will refresh all processes.

2.Unmount Your SD Card
Normally WhatsApp related files are stored in SD card. Unmount your card, remove it and then reinsert it after waiting for a couple of minutes.

3.Delete WhatsApp folder
Backup the contents on your "WhatsApp" folder, delete the folder and check whether you can download image concerned. If yes, copy the contents back into the whatsApp folder.

If you're facing problems while downloading files on the memory card I'll suggest you to go through the following steps that are mentioned below:-

1. Check that whether there is free space available in your memory card or not as if there is no free space you can't download the file from Whatsapp.

2. Check that the file of Whatsapp is present of not. If the file is not present in your SD card then unmount the SD card and after that mount and restart your smartphone again.

3. Whatsapp don't provide any settings in the smartphone so that you can change to the desired memory location.

4. One reason can be the date and time settings of your phone. Check that you've set the correct date and time on your smartphone or not. If it's not set to the current data and time then change to the current data and time.

5. At last if you are unable to download the files on your memory card then format the memory card and then reboot your phone again.