Hi,
I try to change from SD to SPIFlash and all is working fine so far. Absolutly great Job!
I try to load some picture (bmp) into the flash. I imagine to convert the image content in a string and then store into the flash.
think this is basically possible
Open requested file on SD card
 if ((bmpFile = SD.open(filename)) == NULL) {
  Serial.print("File not found");
  return;
 }
Convert to string
String SD_Read;
 while (bmpFile.available()) {
  char ltr = bmpFile.read();
  SD_Read += ltr;
 }
Store to flash
 while (!flash.eraseSector(17, 0));
 if (flash.writeStr(17, 0, SD_Read)) {
  Serial.println("SAVED");
 } else {
  Serial.println("ERROR");
 }
then to display on the TFT, I have to read from Flash convert from String to File.
Did some one try this already?