Is JPEG photo same as RGB JPG file?

I assume they mean an image file with an RGB color profile, such as the sRGB profile in your photo. An actual RGB encoded JPEG file is not very common. You should be able to submit the photos as is.

JPEG files are usually encoded from an RGB source image into a YCbCr intermediate before they are compressed, then when decoded are rendered back to RGB. YCbCr allows the brightness component of the image to be compressed at a different rate than the color components, which allows for a better compression ratio.

An RGB native JPEG compresses the RGB source directly without an intermediate format, which results in a larger file size for the required quality setting, so native format compression would only used in cases where the quality is set to maximum, and even then few compressors actually do this.

The color profile allows the decoder to display the image with the same colors as the source data, which in your case is sRGB, generally used with standard 24-bit color on computer monitors, digital cameras, web cams, and home printers. AdobeRGB is also used, usually with professional images, which renders a more true green on devices that can support it. JPEG can also compress grayscale and CMYK. CMYK images are usually converted to YCbCrK intermediate before compression, but like RGB can be compressed without conversion.

The color profile is not actually part of the JPEG compression standard, but part of the interchange formats (JIF, JFIF, EXIF) that specify how .jpg files are formatted, and how they are displayed. A file without a color profile is assumed to be rendered as sRGB after it is decompressed and decoded.

You can upload your sRGB photos to iStock as they are without converting them.

From an answer by BMPix at iStock Forums - Adobe RGB or sRGB?

Adobe RGB or sRGB?

There has been a lot of discussion about this. What it comes down to is that sRGB must be used for the web, and can produce acceptable prints, but Adobe RGB can sometimes give slightly better print quality, but is totally unsuitable for web use. If you want your photos to look good on iStock, or if your main customers are web designers, sRGB is your best option. If your customers are in the printing market then Adobe RGB is the answer.