Yup I forgot to pack the structs. This fixes things. Oops:
typedef struct __attribute__((__packed__)) {
unsigned char fileMarker1;
unsigned char fileMarker2;
unsigned int bfSize;
uint16_t unused1;
uint16_t unused2;
unsigned int imageDataOffset;
} FILEHEADER;
typedef struct __attribute__((__packed__)) {
unsigned int biSize;
int width;
int height;
uint16_t planes;
uint16_t bitPix;
unsigned int biCompression;
unsigned int biSizeImage;
int biXPelsPerMeter;
int biYPelsPerMeter;
unsigned int biClrUsed;
unsigned int biClrImportant;
} INFOHEADER;
typedef struct __attribute__((__packed__)) {
unsigned char b;
unsigned char g;
unsigned char r;
} IMAGE;