With your suggestion, you don't even need a "conversion" step, assuming that the API that you're using uses RGBA byte ordering.
e.g.

Color* Screen = new Color[1024*768];
Uint32* converted = static_cast<Uint32*>(Screen);//no conversion, just telling fibs about the data type, because they're bitwise equivalents anyway.

If the API uses a different byte-ordering, you can just rearrange the order of the members in your struct, e.g.

struct Color
{
#if defined(API_ARGB)
	   Uint8 alpha;
	   Uint8 red;
	   Uint8 green;
	   Uint8 blue;
#elif defined(API_BRGA)
	   Uint8 blue;
	   Uint8 green;
	   Uint8 red;
	   Uint8 alpha;
#else
#error "todo - more options"
#endif
};

Also, is there a common, generic way to represent images that isn't API specific?

Yep, an array of bytes, like you're suggesting ;)

With your suggestion, you don't even need a "conversion" step, assuming that the API that you're using uses RGBA byte ordering.
e.g.

Color* Screen = new Color[1024*768];
Uint32* converted = static_cast<Uint32*>(Screen);//no conversion, just telling fibs about the data type, because they're bitwise equivalents anyway.

If the API uses a different byte-ordering, you can just rearrange the order of the members in your struct, e.g.

struct Color
{
	   Uint8 alpha;
	   Uint8 red;
	   Uint8 green;
	   Uint8 blue;
};

Also, is there a common, generic way to represent images that isn't API specific?

Yep, an array of bytes, like you're suggesting ;)

With your suggestion, you don't even need a "conversion" step, assuming that the API that you're using uses RGBA byte ordering.
e.g.

Color* Screen = new Color[1024*768];
Uint32* converted = static_cast<Uint32*>(Screen);//no conversion, just telling fibs about the data type, because they're bitwise equivalents anyway.

If the API uses a different byte-ordering, you can just rearrange the order of the members in your struct, e.g.

struct Color
{
	   Uint8 alpha;
	   Uint8 red;
	   Uint8 green;
	   Uint8 blue;
};