Abstract Being the most prominent HI line, Lyα permeates the cosmic web in emission. Despite its potential as a cosmological probe, its detection on large scales remains elusive. We present a new methodology to perform Lyα intensity mapping with broad-band optical images, by cross-correlating them with Lyα forest data using a custom one-parameter estimator. We also develop an analytical large-scale Lyα emission model with two parameters (average luminosity 〈LLyα〉 and bias be) that respects observational constraints from QSO luminosity functions. We compute a forecast for DECaLS/BASS g-band images cross-correlated with DESI Lyα forest data, setting guidelines for reducing images into Lyα intensity maps. Given the transversal scales of our cross-correlation (26.4 arcmin, ∼33 cMpc/h), our study effectively integrates Lyα emission over all the cosmic volume inside the DESI footprint at 2.2 < z < 3.4 (the g-band Lyα redshift range). Over the parameter space (〈LLyα〉, be) sampled by our forecast, we find a 3σ of large-scale structure in Lyα likely, with a probability of detection of 23.95% for DESI-DECaLS/BASS, and 54.93% for a hypothetical DESI phase II with twice as much Lyα QSOs. Without a detection, we derive upper bounds on 〈LLyα〉 competitive with optimistic literature estimates (2.3 ± 1 · 1041 erg/s/cMpc3 for DESI, and ∼35% lower for its hypothetical phase II). Extrapolation to the DESI-Rubin overlap shows that a detection of large-scale structure with Lyα intensity mapping using next-generation imaging surveys is certain. Such detection would allow constraining 〈LLyα〉, and explore the constraining power of Lyα intensity mapping as a cosmological probe.