I have CHIRP allowing the frequencies, but it will only save as a CSV, not an IMG now ... and removes the option to upload to the radio.
All as I can guess is that you are doing something wrong or missing a step.
Just for the fun of it, I RESET my radio. That leaves it with only the firmware default channel 0 and channel 127 programmed. I also have the VHF low and high band limits set at 100 and 199 respectively and the UHF low and high band limits set to 200 and 999 respectively.
At the radio I have selected VFO mode and the top (A) display (required for manually programming memory channels).
I key in [1][2][1][5][0][0] for 121.500 MHz. This frequency is well below the factory VHF low limit of 136.000 MHz and out-of-range for entering into the CHIRP spreadsheet memory editor. It is displayed in the VFO as 121.500.
I then key [MENU][2][7][MENU][0][0][1][MENU] to save the VFO frequency into memory channel 1.
Next I switch to MR (channel) mode and select channel 1. There is my newly programmed memory channel.
Back at my computer using CHIRP, I download from my radio and see my newly added channel 1 with a frequency of 121.500 MHz showing in the spreadsheet memory editor. I then save the "tab" to a file called "BFB231 121.500.img".
Then for the fun of it, I RESET my radio again and uploaded my saved .img file to it. Memory channel 1 with a frequency of 121.500 MHz was restored as expected.
I am using CHIRP daily-20140913 and a very old UV-5R with BFB231 firmware.
Jim KC9HI