How does the frequency of a visual stimulus affect the steady-state visually evoked potential?
Short answer
SSVEP frequency corresponds to stimulus frequency and SSVEP amplitude decreases with stimulus frequency.

Background
The simplest form of steady state visual-evoked potentials (SSVEPs) are those to flash stimuli. Herrmann (2001) studied the SSVEP by recording the EEG from the visual cortex at various flash frequencies. He found that SSVEPs occurred at a frequency corresponding to the flicker stimulus. Amplitudes decreased steadily with frequency and SSVEPs could be recorded up to ~90 Hz flicker frequencies. The shape and frequency following of SSVEPs are shown in Fig. 1.  


Fig. 1. SSVEPs and FFT spectra at various frequencies. Source: Herrmann (2001) 

Interestingly, SSVEPs in response to some frequencies showed resonance phenomena, indicating a selective frequency preference of the neural oscillators. Specifically, SSVEPs at around 10, 20, 40 and 80 Hz resulted in higher amplitudes than stimuli flickering at adjacent frequencies (Fig. 2).


Fig. 2. SSVEPs showing resonance at 80 Hz. Source: Herrmann (2001) 

Reference
Herrmann, Exp Brain Res (2001); 137: 346–353